The present invention relates generally to a thermal ink jet printer used to form images on a recording medium and, more particularly, to a storage container for storing an ink jet printhead cartridge during periods of non-use.
Ink jet printers, or plotters, of the so-called "drop-on-demand" type have at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of channels and energy pulses are applied to transducers to cause the droplets of ink to be expelled, as required, from nozzles at the ends of the channels.
In a piezoelectric printer of the type disclose, for example, in U.S. Pat. No. 5,365,645, the transducers are piezoelectricity deflectable plates. In a thermal ink jet printer of the type disclosed, for example, in U.S. Re. 32,572, the transducers are resistors, which are individually addressable by current pulses to heat and vaporize ink in a channel or recess proximate to the nozzle. As a vapor bubble grows, ink bulges from the nozzles until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel or recess retracts and separates from the bulging ink which forms a droplet moving in a direction away from the nozzles and towards the recording medium. The channel or recess is then re-filled by capillary action, which in turn draws ink from a supply cartridge. Operation of a thermal ink jet printer wherein the ink is expelled from channels is described in, for example, U.S. Pat. Nos. 4,638,337 and 4,774,530, which disclose a printer of the carriage type having a plurality of printheads, each with its own ink supply reservoir, mounted on a reciprocating carriage. The nozzles of each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information.
It is necessary to periodically store an ink jet printhead cartridge in a humid environment during periods of non-use. The storage medium most typically includes a capping mechanism for sealing the printhead nozzle face to obtain an airtight seal for the nozzles to inhibit evaporation of ink within the ink channels. U.S. Pat. No. 5,373,936 discloses a storage container which includes a pumping mechanism for applying negative pressure to remove ink from around the ink nozzles while applying a capping mechanism in sealing engagement with the nozzle face.
A continuing problem with prior art capping mechanisms is the tendency of the sealing edges of the nozzle face capping mechanism interface to buckle and allow exposure to the external environment (also called seal loss) because of the applied sealing force.
The problem is shown with reference to FIG. 1, which shows in schematic form a portion of a prior art storage container 8, which incorporates a ribbed, low durometer (20.+-.5 shore "A") capping membrane 10 typically made of a material such as silicone. A printhead cartridge 12 includes an ink tank 14 filled with ink 15 fluidly connected to a printhead 16 having a plurality of channels terminating at nozzles in a nozzle face 18. Capping membrane 10 includes a raised rib portion 11 which assumes a generally rectangular orientation with an area which encompasses the nozzles of nozzle face 18. Member 10 also has a base portion 13. The cartridge 12 must be seated within a container such that the nozzle face 18 is in sealing contact against base 13 with ribbed portion 11 providing the edge seal. The cartridge 12 is moved so as to create a force great enough to effectively seal nozzle face 18 but not so strong as to cause buckling in rib portion 11. This buckling tends to occur as the compression load increases and the rib section can no longer withstand the compression. This buckling tends to occur over long sections, but not at the corners where the strength is greater.
The applied sealing force is large enough to compensate for non-planarity of the nozzle face surface. The design latitude for such a prior art sealing mechanism is typically quite narrow and requires a relatively large sealing force to be applied.
Another problem with the prior art design of FIG. 1 is that when the nozzle face is seated, an air pocket can be compressed creating a positive pressure buildup around the nozzle face which can exceed 1 psi. At this pressure, the printhead begins to deprime with ink being forced back into the ink channels. Also, while the printhead remains capped, temperature changes can also increase the positive pressure. When the printhead is removed, the release of the positive pressure can result in ink weeping from the nozzles.